Significant quantities of rarer elements tend to collect in intermediate refinery residues, sludges and dusts formed during the processing of ores, concentrates, mattes, etc., for recovery of their major valuable components. Minor metal components also collect with residual amounts of the major elemental components and are recovered from sludges accumulating in sulfuric acid plants. By refinery residues is meant materials such as anode slimes produced in the electrolytic refining of copper and nickel, accumulated impurities from the carbonyl treatment of nickel mattes to recover essentially pure nickel, dusts from roasting and smelting operations. While such residues vary widely in composition, they generally contain significant amounts of copper, selenium, tellurium, lead, silver, gold and some platinum group metals along with heavy metal nuisance elements such as arsenic, antimony, bismuth, tin and lead. Other elements that may be present are nickel and iron. Gangue components such as Al.sub.2 O.sub.3, SiO.sub.2, CaO are also usually present in the residues. The present process may also be used to separate metal values from other materials, such as precious metal catalysts that may have become contaminated during use. It will be apparent that whether a metal component is considered a major or minor component or an impurity depends on such things as concentration, ease of recovery, and economics with respect to precious metals, however, even when present as slight impurities, they may be cumulatively of great value when isolated and their presence may control the processing method the refiner selects.
Another determinative factor in treating residues for recovery of metals involves environmental considerations. For example, pyro- and vapormetallurgical steps may result in varying degrees of undesirable emissions containing, for examples, oxides of selenium, tellurium, sulfur, lead, and other heavy metals. Thus it is highly desirable to treat materials containing such metals by a route which reduces the amount of smelting operations, avoids steps which are most objectionable, and preferably is totally hydrometallurgical.
The present invention is described with particular reference to the treatment of anode slimes formed in the electrolytic refining of copper and nickel. Typical compositions of copper refinery slimes are given on pages 34-35 of SELENIUM edited by Zingaro. R. A. and Cooper, W. C., Van Nostrand Reinhold Company (1974). Approximate ranges (in wt. %) of selenium, tellurium, copper, nickel, lead, and precious metals are as follows: 2.8 to 80% copper, 1 to 45% nickel, 0.6 to 21% selenium, 0.1 to 13% tellurium, 1 to 45% silver, 0.3 to 33% lead, up to 3% gold and minor amounts platinum group metals. Gangue components such as Al.sub.2 O.sub.3, SiO.sub.2 and CaO are present in the amount of about 2 to 30%.
Generally, in conventional processes the anode slimes are first sequentially treated for the removal of copper, nickel, selenium and tellurium. One of the particularly difficult problems is the extraction of silver and other precious metals, which may be bound up in the slimes and at intermediate processing stages in compounds with selenium and/or tellurium. One widely used technique for the recovery of precious metals from slimes is to form a Dore metal, which is a precious metal ingot obtained by smelting the residue previously treated for the removal of copper, nickel, selenium and tellurium. The Dore metal is electrorefined for silver recovery, and the slimes obtained in electrorefining of silver can be further treated for the recovery of gold and platinum group metals. Dore smelting, however, is often regarded as the most expensive and complicated step of slimes treatment processes. Also, it can produce harmful emissions, e.g., of selenium, arsenic, lead and antimony oxides. In co-pending U.S. application Ser. No. 26,302, a method is disclosed for treating anode slimes and similar types of materials for the recovery of valuable components, particularly silver by a hydrometallurgical technique.
In accordance with the aforesaid pending application materials such as anode slimes are treated by a method comprising: converting silver values comprising silver compounds of selenium and/or tellurium to a material containing silver in a form readily leachable in dilute nitric acid, leaching such silver-containing material with dilute nitric acid, and recovering silver from such leach solution by electrowinning. Preferably the silver values are converted to at least one of the species elemental silver, a silver oxide and silver carbonate. Silver sulfide is a less desirable species since it is not as readily converted to the nitrate. Depending on various factors such as the composition of the feed, cost, location and availability of reagents and fuel, different processing routes may be taken to separate silver from other valuable components and to remove one or more impurities. The pretreatment route is not critical to the invention so long as the silver species obtained is leachable in dilute nitric acid. Preferably the overall process is hydrometallurgical and the initial treatments may be in an acid or base medium, as explained more fully in the co-pending application.
Many methods for separating and recovering various other components from the slimes have been proposed. For example, U.S. Pat. No. 4,163,046 discloses a hydrometallurgical route for the recovery of commercially pure selenium involving a caustic oxidative pressure leach, neutralization, sulfide treatment and acidification to obtain an essentially precious metal-free, tellurium-free selenium solution from which selenium is precipitated using SO.sub.2 in the presence of the alkali metal halide and ferrous ion.
U.S. Pat. No. 2,981,595 shows a step in a process for recovery of tellurium from slimes in which a sulfuric acid solution containing copper and tellurium in sulfate form is treated with metallic copper to cement tellurium from the solution. It is also known to separate silver from copper and from lead and other elements such as antimony and arsenic by the use of chlorine gas. U.S. Pat. No. 712,640 uses this technique for the treatment of anode residues produced in the electrolytic refining of lead. It has also been shown that gaseous chlorine breaks down slimes constituents in aqueous medium at room temperature. Acid oxidative pressure leaching of raw slimes is one of the known techniques for separating selenium and tellurium. At an AIME Meeting in 1968 a hydrometallurgical method was reported for treating copper refinery slimes included a pressure leach of slimes in dilute sulfuric acid at 110.degree. C. under 50 psi oxygen pressure to dissolve all of the copper and most of the tellurium, with cementation of the tellurium from solution with copper shot.
While each of the techniques mentioned above has useful aspects, none of them or processes which employ such technique is completely satisfactory. Problems arise not only because of the requirements, e.g. desired purity of particular end products, but also because of compositional peculiarities of the residues which are treated.
In the present method the material treated contains selenium, silver and also contains at least one other precious metals other than silver, such as gold or a platinum gold metal, e.g. platinum, palladium rhodium and ruthenium, and at least one nuisance element such as bismuth, lead, tin, arsenic and antimony. As indicated above, the material may also contain copper, nickel, tellurium, and gangue minerals such as SiO.sub.2 or Al.sub.2 O.sub.3. One of the problems in treating such materials is the separation of the nuisance elements from the more valuable components in an environmentally sound manner. Where the levels of palladium and/or platinum are high, difficulties arise if these metals report to the silver electrowinning phase of the process.
It is an object of the present invention to treat precious metal containing streams which also contain selenium and nuisance elements to separate the component elements in an environmentally sound manner. A further object is to carry out the overall process for recovery of such components using hydrometallurgical techniques. Another object is to separate nuisance elements from precious metals in a simple effective hydrometallurgical manner. Still another object is to separate and recover selectively selenium, platinum group metals, gold and silver from material which also contains nuisance elements. A further object is to recover a large fraction of the gold in the feed in substantial pure form and to recover selenium and/or tellurium in forms suitable for commercial sale. Another object is to achieve high recoveries of the metal values.